1. Field of the Invention
The present invention relates to a photochromic optical disk read/write apparatus for writing and reading of data signals to and from a photochromic recording medium.
2. Description of the Related Art
Organic photochromic material is known as a chemical compound capable of coloring and discoloring in response to light emissions. More particularly, as shown in a spectral distribution diagram of light absorption of FIG. 2, such an organic photochromic material changes from the stable state X to the metastable state Y for putting a color on when absorbing an intensity of light having a waveform B and when absorbing at the metastable state Y a different light or heat having a waveform A, will turn back to the original state and become discolored. With the application of such a reversible optical or thermal isomeric reaction, an EDRAW disk for recording at relatively higher density will be eligible.
A read/write apparatus for recording and reproducing information signals to and from such a photochromic optical disk is disclosed in Japanese Patent Application 01-199643 (1989).
That read/write apparatus employs laser beams of 350-nm wavelength for write operation and of 632.8-nm wavelength for read or erase operation. The photochromic optical disk has tracks provided on the primary surface thereof. The tracks are specified by a series of pregrooved surface shaping variations or prepits which represent address information of data blocks (thus, will be termed as "prepits" hereinafter) and are separated by given angles into sections spaced at radially equal distances. Then, arranged along the track are a row of coloring regions (referred to as "coloring pits" or "data pits" hereinafter) of a photochromic recording layer for optical recording of information signals.
For use in tracing of the track, an eligible tracking beam is prepared by lowering in the intensity the light beam for reading and erasing at the read and erase mode and the light beam for writing at the write mode.
The prepits are thereby detected by a relatively lower intensity of light beam for tracking. However, the exposure to the lower beams after repeated tracking actions will cause gradual declination in the quality of a recorded or erased state. Particularly, tracing of the same track during the pause action will accelerate deterioration of the recorded or erased state quality.